In recent years, with the advance of the information society, there has been a growing demand for a large-capacity external storage. For optical information recording, an increase in density by reducing the size of recording pits has been restricted due to a diffraction limit that depends on the wavelength of light and the numerical aperture of an objective lens.
Therefore, a so-called super-resolution recording/reproducing technology that enables readout of a recording mark smaller than a focusing spot diameter has been proposed as a means for achieving higher density. For example, JP 2000-348377 discloses a technique that uses near-field light to perform recording/reproduction beyond the diffraction limit of light.
FIG. 8 shows the cross-sectional configuration of an optical recording medium used in super-resolution recording/reproduction. This optical recording medium 1 includes a transparent substrate 11 and a first protective layer 12, a mask layer 13, a second protective layer 14, a recording layer 15, and a third protective layer 16 that are formed on the transparent substrate 11 in the indicated order. The first to third protective layers 12, 14, and 16 are made of ZnS—SiO2. The recording layer 15 is made of a phase change material (e.g., a multinary compound such as GeSbTe). The mask layer 13 is made of silver oxide that is decomposed into oxygen and silver by heat. When the optical recording medium 1 is irradiated with convergent light L1, a focusing spot is formed in the mask layer 13, and then the silver oxide is decomposed into oxygen and silver to change the refractive index in a high-temperature portion of the focusing spot where the temperature exceeds a given threshold value. Thus, an aperture 17 smaller than the focusing spot diameter is formed in the mask layer 13 as a refractive index changing region. It is possible to write a recording mark 18 into the recording layer 15 or read the recording mark 18 from the recording layer 15 using near-field light generated at the aperture 17. The recording layer 15 is located at the position where the near-field light generated in the mask layer 13 can reach, thereby achieving both high-speed writing and high-speed reading.
A general optical head device has been used in the above super-resolution recording/reproduction. FIG. 9 shows a general optical head device 101 when used for reproducing information from the optical recording medium 1. For convenience, the lateral direction of the sheet of this drawing is identified as an X-direction, the vertical direction from the sheet surface is identified as a Y-direction, and the longitudinal direction of the sheet is identified as a Z-direction.
A semiconductor laser 102 (a radiation light source) radiates linearly polarized light that is polarized in the X-direction. The light emitted from the semiconductor laser 102 enters a polarization beam splitter 103. The polarization beam splitter 103 has the functions of transmitting all the light polarized in the X-direction and reflecting all the light polarized in the Y-direction. The light passing through the polarization beam splitter 103 is converted into parallel light by a collimator lens 104, then converted into circularly polarized light by a quarter-wave plate 105, and focused to the inside of the optical recording medium 1 by an objective lens 106. The light reflected from the optical recording medium 1 again passes through the objective lens 106 and the quarter-wave plate 105, and thus is converted into linearly polarized light that is polarized in the Y-direction. This linearly polarized light further passes through the collimator lens 104 and enters the polarization beam splitter 103. The light entering the polarization beam splitter 103 is polarized in the Y-direction, and therefore is reflected by the polarization beam splitter 103. The reflected light passes through a cylindrical lens 107 or the like so that the wavefront is controlled to detect a servo signal. Subsequently, a photodetector 108 detects a reproduction signal and a servo signal. The quarter-wave plate 105 and the polarization beam splitter 103 are used to improve the light utilization efficiency. Even if the quarter-wave plate 105 is not provided, and the polarization beam splitter 103 is replaced with a non-polarization beam splitter, information can be recorded/reproduced.
When optical information is reproduced in the above manner, a general laser beam includes a noise component. For normal reproduction (rather than the super-resolution reproduction), the noise component is not a problem because the degree of signal modulation is sufficient. In the case of super-resolution reproduction, however, a recording mark smaller than the spot diameter of the laser beam should be read, so that the degree of modulation of a reproduction signal is reduced significantly. Therefore, the effect of the noise component of the laser beam cannot be ignored, which may lead to S/N degradation.